This invention relates to a light beam scanning apparatus for causing, for example, laser beams to scan the surface of a photosensitive drum simultaneously to form an electrostatic latent image on the photosensitive drum and to an image forming apparatus, such as a digital copying machine or a laser printer, using the light beam scanning apparatus.
In recent years, various digital copying machines have been developed which form images by, for example, laser-beam scanning exposure and electronic photograph processing.
To step up the image forming speed, the multi-beam digital copying machines have recently been developed. In this type of digital copying machine, more than one laser beam is generated and they are caused to scan in units of lines simultaneously.
The multi-beam digital copying machine comprises semiconductor laser oscillators for generating laser beams, a polyhedral rotating mirror, such as a polygon mirror, and an optical system unit acting as a light-beam scanning apparatus. The polyhedral rotating mirror reflects the laser beams emitted from the laser oscillators toward a photosensitive drum to cause each laser beam to scan the surface of the photosensitive drum. The optical system unit is composed mainly of a collimator lens and an f-xcex8 lens.
A method of controlling the exposure position accurately in the direction in which a laser beam scans (or the main scanning direction) in a digital copying machine of the multi-beam type has been disclosed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 1-43294, Jpn. Pat. Appln. KOKOKU Publication No. 3-57452, Jpn. Pat. Appln. KOKOKU Publication No. 3-57453, Jpn. UM Appln. KOKOKU Publication No. 5-32824, or Jpn. Pat. Appln. KOKAI Publication No. 56-104572.
Jpn. Pat. Appln. KOKOKU Publication No. 1-43294 has disclosed a method of using a light beam sensor to sense the timing with which light beams arrive. In this method, the order in which the light beams arrive is unknown. Therefore, the method is not suitable for an optical system where two or more light beam arrive simultaneously.
Jpn. Pat. Appln. KOKOKU Publication No. 3-57452 has disclosed a method of providing separate light-receiving sections for sensing light beams independently and permitting each light beam to expose the corresponding light-receiving section and pass through it. On the basis of the signal from each light-receiving section, the light-emitting timing for printing by each light beam (or recording or image formation) is produced.
However, for example, when plural recording dot pitches (resolutions), such as 300 dpi, 400 dpi, and 600 dpi, or 16 lines/mm and 15.4 lines/mm, are needed, the number of revolutions of the polygon mirror or the frequency of image clock must be changed. In this case, it is difficult to align the print start position of each light beam because of the following problems: the phase of the output signal from each light-receiving section may change with respect to the corresponding light beam, the timing with which each light beam arrives at the print start position, and the difference in arriving timing between the light beams cannot be divided by one period of image clock.
The method disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 3-57453 has been based on the assumption that the main scanning image formation area for each light beam is designed to shift in the main scanning direction. Therefore, the method is not suitable for such an optical system as is shown in embodiments of the present invention.
The method disclosed in UM Appln. KOKOKU Publication No. 5-32824 is not suitable for such an optical system as is shown in embodiments of the present invention because of the same reason as in the method in Jpn. Pat. Appln. KOKOKU Publication No. 3-57452.
The method disclosed in Jpn. Pat. Appln. KOKAI Publication No. 56-104572 is to produce a synchronizing signal using one of plural light beams and control the light-emitting timing for each light beam on the basis of the synchronizing signal. The relationship between the scanning positions of the light beams must be known beforehand. Therefore, the method is not suitable for such an optical system as is shown in embodiments of the present invention.
It is an object of the present invention to provide a light beam scanning apparatus and an image forming apparatus which are applicable to an optical system where the order in which light beams are caused to scan in the main scanning direction is unknown (they may be caused to scan simultaneously) and which are capable of constantly controlling the exposure position in the main scanning direction with an accuracy of a small fraction of a pixel.
Another object of the present invention is to provide a light beam scanning apparatus and an image forming apparatus which are applicable to more than one resolution.
In order to achieve the above objects, according to one aspect of the present invention, there is provided a light beam scanning apparatus comprising: a plurality of beam generating means for generating light beams scanning means for optically combining the light beams generated at the beams; generating means, reflecting the combined beams to a scanning surface including the surface of an image retaining element, and causing the light beams to scan the scanning surface; first sensing means, provided near the image retaining element for sensing the first one of the light beams caused by the scanning means to scan; clock generating means for each of the light beams, in response to a sensed signal indicating that the first beam has exposed the first sensing means; second sensing means, provided on the more downstream side in the main scanning direction than the first sensing means, and for sensing the light beams; first control means for giving control data to the clock generating means so that the light beams may expose a target area between the first and second sensing means and image formation area setting means for determining a pixel clock area corresponding to a target image formation area on the image retaining element on the basis of the control data from the control means and setting the pixel clock area in the clock generating means.
Accordingly, four beams, which are parallel with each other in the sub-scanning direction, are caused to scan a scanning surface in the main scanning direction. The beams expose the target area between two optical sensing means, not the surface of the image retaining element, to produce correction data to correct shifts in the exposure positions of the four beams in the main scanning direction with an accuracy of less than a small fraction of one pixel. On the basis of the correction data, an image formation area is set on the image retaining element.
The clock generating means includes: clock means for generating a clock signal a specific time after the first beam exposed the first sensing means; delay means which delays the clock signal generated at the clock means, selects the delay of the clock in a range of one clock or less for each beam, and provides a delayed clock signal as a pixel clock to be used to generate a beam; and clock setting means for setting, for the respective beams, exposure pixel clock areas used by the beam generating means in the pixel clocks given by the delay means and providing exposure pixel clocks.
The delay means includes a delay line and delay clock selectors, the delay line having taps, and each of the delay clock selectors designed to select and output a delayed clock generated at one tap, which is to be used to generate one beam.
Specifically, to align the image formation positions of the four beams accurately in the main scanning direction, the amount of delay is given to the clock signal with an accuracy of one-tenth of a clock for each of the beam generating means independently.
The target area between the first and second sensing means is an area overlapping with the second sensing means, the light beam scanning apparatus further comprising: a counter for counting the number of times the beams expose the second sensing means; second control means for driving one of the beam generating means using the exposure pixel clock obtained from the clock setting means controlled on the basis of the control data and causing the scanning means to scan the scanning surface a specific number of times; comparison means for reading a value in the counter after the second control means has scanned the specific number of times and comparing the value with a predetermined number; means for selecting the next tap whose amount of delay is greater than that of the selected tap when the result of the comparison at the comparison means has shown that the value in the counter is smaller than the predetermined number; and amount-of-delay setting means, when the result of the comparison at the comparison means has shown that the value in the counter is equal to the predetermined number, for setting a delay selected at that time as the amount of delay for the one of the beam generating means.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.